Image intensifier tubes are used in night/low light vision applications to amplify ambient light into a useful image. A typical image intensifier tube is a vacuum device, roughly cylindrical in shape, that generally includes a body, photocathode and faceplate, microchannel plate (MCP), and output optic and phosphor screen. Incoming photons are focused on the glass faceplate by external optics, and strike the photocathode which is bonded to the inside surface of the faceplate. The photocathode or cathode converts the photons to electrons, which are accelerated toward the input side or electron-receiving face of the MCP by an electric field. The MCP has many microchannels, each of which functions as an independent electron amplifier, and roughly corresponds to a pixel of a CRT. The amplified electron stream emanating from the output side or electron-discharge face of the MCP excites the phosphor screen and the resulting visible image is passed through the output optics to any additional external optics. The body holds these components in precise alignment, provides electrical connections, and also forms the vacuum envelope.
Conventional MCPs are formed from the fusion of a large number of glass fibers, each having an acid etchable glass core and one or more acid-resistant glass cladding layers, into a solid rod or boule. Individual plates are sliced transversely from the boule, polished, and chemically etched. The MCPs are then subjected first to a hydrochloric acid bath that removes the acid etchable core rod (decore), followed by a hot sodium hydroxide bath that removes mobile alkali metal ions from the glass cladding.
The microchannels of a MCP are typically cylindrical in shape and inclined 1-10.degree. from normal to the MCP surface, The ratio of the combined channel area to the overall active area of the MCP is known as open-area-ratio (OAR). The ratio of the core rod area to overall area prior to acid etch is approximately 45%.
Acid etching (decore) of the MCP increases OAR to 55%, and the alkaline (e.g. sodium hydroxide) leach step further increases the OAR to approximately 60% by alkali removal of acid-resistant cladding.
Unfortunately, the output image of conventional MCPs is degraded, i.e., the output image is not a perfect replica of the input image. Degradation of the output image is caused by a number of factors. For example, a photoelectron released from the photocathode may not fall into one of the microchannels of a conventional MCP but, will impact the flat face of the MCP in a region between the openings of the microchannels. Electrons that hit the flat face are likely to be deflected or bounce back toward the photocathode before being directed back to the MCP by the electrostatic field. These bounced photoelectrons produce a number of secondary electrons from a part of the MCP that is not aligned with the proper location of photocathode generation. Such an alignment problem results in both decreased signal-to-noise ratio, and distortion of the image produced by the image intensifier, i.e., halos in the output image. At other times the errant electron is simply absorbed by the face of the plate and is not further amplified, thus reducing the signal-to-noise ratio.
One solution to this problem is to increase the open-area ratio by using fibers with thinner acid resistant cladding layers relative to the acid soluble cores. However, a MCP having parallel walls is limited to an open area ratio of approximately 65% so that 1) sufficient glass remains for structural integrity, and 2) no deleterious electric field effects appear. These latter electric field effects arc induced by spike structures resulting from close packed, thin clad fibers after acid etching.
Accordingly, there remains a need for an improved MCP which overcomes the disadvantages of conventional MCPs.